There was previously proposed, as an improved permanent magnet of high performance which exceeded the highest magnetic properties of the conventional rare earthcobalt magnet, an Fe-B-R type permanent magnet, which was composed as the principal components of iron (Fe), boron (B) and light rare earth elements such as neodymium (Nd) and praseodymium (Pr) abundantly available in natural resources, and which was free of expensive samarium (Sm) or cobalt (Co) (Japanese Patent KOKAI Publications Nos. 59-46008 (1984) and 59-89401 (1984) or EPA 101552).
The alloy of the above mentioned magnet has the Curie temperature which is usually in the range from 300.degree. to 370.degree. C. However, another Fe-B-R type permanent magnet having a higher Curie temperature may also be prepared by substituting cobalt (Co) for a part of iron (Fe) (Japanese Patent KOKAI Publications Nos. 59-64733 (1984) and 59-132104 (1984) or EPA 106949).
With a view to improving the temperature characteristics, in particular the coercivity iHc, and retaining however the Curie temperature equal to or higher than and a (BH)max higher than the above mentioned Co-containing Fe-B-R type (i.e., (Fe, Co)-B-R type rare earth permanent magnet, there was also proposed still another Co-containing Fe-B-R type rare earth permanent magnet in which at least one of heavy rare earth metals such as dysprosium (Dy) or terbium (Tb) is included as a part of the rare earth elements (R), whereby the coercivity iHc was improved further with the (BH)max remaining at an extremely high level of not less than 200 kJ/m.sup.3 (25 MGOe (Japanese Patent KOKAI Publication No. 60-34005 (1985) or EPA 134304).
However, the permanent magnet formed by the Fe-B-R type magnetically anisotropic sintered body, while exhibiting the above mentioned excellent magnetic properties, has the contents of the rare earth elements and iron, that are apt to be oxidized in air to form gradually stable oxides, as the main constituents, so that, when the magnet is assembled in the magnetic circuit, various problems may be invited due to oxides formed on the magnet surface, such as decreased output of the magnetic circuit, fluctuations in the operation of the various magnetic circuits and contamination of various peripheral devices around the magnetic circuits due to scaling off of the resultant oxides from the magnet surface.
Therefore, with a view to improving the corrosion resistance of the above mentioned Fe-B-R type permanent magnet, there was already proposed a permanent magnet having a corrosion-resistant metal plating layer formed on the magnet surface by an electroless plating method or by the electrolytic plating method (Japanese Patent Application No. 58-162350 (1983), now KOKAI publication No. 60-54406. However, since the permanent magnet is a sintered porous body, there is a risk with these plating method that an acidic or alkaline solution from the pre-plating operation may remain within the pores to cause corrosion with the lapse of time, and that, since the magnet body is inferior in its resistance to chemicals, the magnet surface may be attacked during plating to cause deterioration in adhesivity and corrosion resistance.
The results of tests on corrosion resistance with the magnet being left for 100 hours under the conditions of the temperature of 80.degree. C. and the relative humidity of 90% have also revealed that the magnetic properties exhibited deterioration of 10% or more from its initial properties and remained extremely unstable. Problems to be solved by the invention
Furthermore, as the Fe-B-R type permanent magnet which could successfully solve the disadvantages inherent in the abovementioned plating method, spraying method and dipping method, and provide stabilized corrosion resistant property over a long period of time, there were also proposed improved permanent magnets provided on its surface with a vapor-deposited corrosion-resistant layer composed of various metals or alloys (Japanese Patent Applications No. 59-278489, No. 60-7949, No. 60-7950 and No. 60-7951, now corresponding EPA 0190461). By this vapor-deposition method, oxidation of the surface of the magnet body is suppressed, so that the magnetic properties are prevented from deterioration. Also, since there is no necessity for corrosive chemicals, etc., hence no apprehension whatsoever of its remaining in the magnet body as is the case with the plating method, the permanent magnet as treated by this method is capable of retaining its stability over a long period of time.
While the vapor-deposition method is effective for improvement in the corrosion resistance of the permanent magnet, it has its own disadvantage such that its productivity is low, so that the treatment by this method is considerably expensive.
In view of the foregoing, the present Applicant has made it clear that, by forming an electroless plating layer composed of at least one noble metal selected from the group consisting of palladium (Pd), silver (Ag), platinum (Pt) and gold (Au) and at least one base metal selected from the group consisting of nickel (Ni), copper (Cu), tin (Sn) and cobalt (Co), by an electroless plating method, on the surface of the above mentioned Fe-B-R type sintered magnet body, the electroless plating layer becomes dense, such that the deterioration of the initial magnetic properties of the permanent magnet may be reduced to not more than 10% in case of changes in the external environment, such as humidity on gases (Japanese Patent Application Nos. 62-73920 (1987), 62-90045, 62-90046 and 62-100980; now corresponding KOKAI Publication Nos. 63-238240, 63-255376, 63-254702 and 63-266020).
However, should the base metal layer be formed by the electroless plating method after the noble metal layer has been formed on the surface of the permanent magnet, adhesivity of the metal layers become inferior, such that, in the above mentioned tests on corrosion resistance, it becomes occasionally not possible to reduce the deterioration of the initial magnetic properties to 5% or less.
In addition, should the base metal layer be formed by the electrolytic plating method after the noble metal layer is formed on the surface of the permanent magnet, a tough metal coating would be obtained. However, in such case, the rare earth elements as the constituents of the magnet tends to be solved into the plating solution from the surface of the sintered magnet body to cause the corrosion to start from the interior of the magnet body.